Remarkable progress in lasers have been made in recent years. In particular, solid lasers and semiconductor lasers which emit infrared rays having a wavelength of 760-1,200 nm (hereinafter often referred to as “infrared lasers”) have come to be easily available as high-output small apparatus. Especially in the field of lithography, these infrared lasers are exceedingly useful as recording light sources in direct platemaking based on digital data from, e.g., a computer. With the progress of lasers, various investigations are being made on image-recording materials sensitive to such various laser lights. A positive type recording material (see U.S. Pat. No. 4,708,925) and a negative type recording material crosslinking with an acid catalyst (see JP-A-8-276558) are known which are sensitive to a light having a wavelength of 760 nm or longer and are usable with an infrared laser.
A negative type image-recording material in which recording is possible with exposure to infrared rays (infrared-sensitive recording material) generally comprises an infrared absorber which absorbs the energy of infrared light and converts it into heat, a radical polymerization initiator which generates radicals by the action of the heat produced by the infrared absorber, and a radical-polymerizable compound which polymerizes with the aid of the radicals generated as an initiator. In lithographic printing plate precursors employing this recording material, recording is conducted by a mechanism in which the polymerization reaction of the radical-polymerizable compound occurs and proceeds to thereby cure exposed areas of the recording layer (image-recording material) and thus form an image. In particular, a lithographic printing plate precursor having a photopolymerizable recording layer containing a binder polymer soluble in alkaline developing solutions besides the ingredients shown above has advantages of excellent productivity, ease of development, satisfactory resolution, and satisfactory ink receptivity and, hence, can give a printing plate having desirable printing performances.
Known as the infrared-sensitive recording material utilizing a radical polymerization reaction is one containing a resin having amide groups in side chains as a binder polymer. However, this recording material is insufficient in printing durability (see JP-A-2001-242612). Furthermore, one containing a vinyl resin having (meth)acryloyl groups in side chains as a binder polymer is insufficient in sensitivity and printing durability (see JP-A-2002-62648), while one containing a polyurethane resin as a binder polymer has had problems of, e.g., insufficient removability of nonimage areas in development (see JP-A-2001-312062).
On the other hand, examples of infrared-sensitive recording materials which are cured by a reaction mechanism other than radical polymerization include an infrared-sensitive recording material comprising a polyazide photoinitiator, an infrared-absorbing dye, and a polyamide resin containing carboxy groups in side chains as a binder polymer. However, this recording material failed to give an excellent cured film through exposure and has been insufficient in both of sensitivity and printing durability (see JP-A-10-115914).